auth-backend: always exchange and never return refresh tokens to clients

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/four-phones-shave.md

@@ -0,0 +1,29 @@
+---
+'@backstage/plugin-auth-backend': minor
+---
+
+Avoid ever returning OAuth refresh tokens back to the client, and always exchange refresh tokens for a new one when available for all providers.
+
+This comes with a breaking change to the TypeScript API for custom auth providers. The `refresh` method of `OAuthHandlers` implementation must now return a `{ response, refreshToken }` object rather than a direct response. Existing `refresh` implementations are typically migrated by changing an existing return expression that looks like this:
+
+```ts
+return await this.handleResult({
+  fullProfile,
+  params,
+  accessToken,
+  refreshToken,
+});
+```
+
+Into the following:
+
+```ts
+return {
+  response: await this.handleResult({
+    fullProfile,
+    params,
+    accessToken,
+  }),
+  refreshToken,
+};
+```